The invention relates to machine elements and mechanisms, but more particularly, the invention relates to V-belts, V-blocks for variable speed belts, embedded reinforcements for V-blocks, a generally flat load carrying section for a V-block belt, and a process for making a V-block belt.
Standard V-belts have a belt width to belt thickness ratio of generally less than 2.0:1. The higher horsepower/torque capacity belts have a width to thickness ratio as close to 1:1 as possible. The low width to thickness ratio ensures adequate support of a spirally wound tensile member from sheaves that entrain the belt.
A variable speed belt is required to operate at various driver and driven sheave diameters to give a variable speed ratio. The change in diameter possible with a V-belt in a sheave having any given groove angle is directly proportional to the bottom width of the belt. A variable speed belt, therefore, should have a high top width to thickness ratio to give a sufficiently large bottom width for a reasonable sheave diameter change. Normally, a variable speed belt has a width to thickness ratio that is greater than 2.25:1. As the belt width increases, the horsepower/torque carrying capacity decreases primarily because the belt receives a lesser percentage of transverse support from its entraining sheaves. Expressed in other terms, the "sag" at the center of a belt for a fixed total tension and bending radius around a sheave, is about proportional to the cube of the belt width.
Early solutions to the variable speed belt transverse stiffness problem were solved by clamping individual V-blocks to a flat belt. Examples of such solutions appear in U.S. Pat. Nos. 2,387,183 and 2,638,007. These patents show a flat belt construction for the load carrying section, with a tensile member disposed in an elastomeric material, and in one case, transverse ribs at the radially inner side of the belt for engaging with clamping V-blocks. In both cases the V-blocks are individually clamped to the belt with threaded fasteners and an upper beam member. A problem with the clamping type V-blocks is that the beam members which clamp the flat belt load carrying section are interconnected with fasteners which create an articulated structure having an inability to effectively distribute substantially uniform power loads across the V-block driving surfaces.
Another solution to the transverse rigidity problem for variable speed belts is solved by embedding a metallic transverse reinforcement that extends below the tensile member and along the driving V sides of the belt. Such an arrangement is disclosed in U.S. Pat. No. 2,189,049 as a means for making a belt splice. However, the belt with a beam member only below the load carrying section has a very low horsepower/torque carrying capability (i.e., because the V-side members have no cross support above the load carrying section).
A common load distribution problem of these type belts is that the diagonal sides of the V-blocks are either not reinforced with a continuous member or that where there is a continuous V-side reinforcement member, it is arranged as a cantilevered beam member. In either case, there is an unequal load distribution across the V-sides which causes repeated rocking of the V-blocks. The rocking usually leads to early fastener failure or substantially reduces the power load capability of the belt.
A common term used by the belt industry to indicate a relative comparison between belts is "horsepower multiplier." The horsepower transmitting capability of a belt in a drive system is affected by belt speed and sheave diameter. For example, increasing the sheave diameter by 35% often doubles the power transmitting capability of a belt for a given life. A small improvement in horsepower/torque (i.e., load) carrying capacity has an enormous effect on belt life. This is why the real test of belt improvement is that of how much greater horsepower/torque can be carried at the same belt life. The ratio of the tested belt horsepower to a horsepower value established for a base line belt (at a given sheave diameter, rpm, and hours life) is referred to as horsepower multiplier. Using this definition and 1968 as the 1.0 index year, variable speed belts would have approximately the following horsepower multiplier relationship over the years: 1943/0.6; 1948/0.8; 1958/0.8; 1968/1.0 and 1978/1.6. It becomes exceedingly harder to double the horsepower multiplier for present day belts because successively doublings in performance for every improved base line belt establishes an exponential performance curve. It has taken approximately 35 years to achieve the 1.6 horsepower multiplier.
Those block belts which are made by clamping V-blocks to a flat belt seem to have a somewhat limited horsepower capability (e.g., a horsepower multiplier of 0.8) perhaps because the clamping arrangements sets up stress concentrations in the flat belt which lead to early failure as the flat belt is cycled in bending as it operates around entraining sheaves. A variable speed belt with a reinforcement disposed below the tensile member, such as disclosed in the U.S. Pat. No. 2,189,049, are also of lower performance (e.g., a 0.2 horsepower multiplier because of the clamping/splicing arrangement.
Technology has improved the variable speed belt's horsepower/torque capabilities. (e.g., the 1.6 horsepower multiplier). The present day high power belt typically has an endlessly wound tensile member that is sandwiched between an overcord section and an undercord section that has an embedded fiber or a plurality of laminated fabric layers that increases the belts transverse stiffness. It should be noted that the modern-day belt has a horsepower/torque carrying capability that is approximately 2.0 times that of the earlier day block belts.
The common problem with all three types of variable speed belts (i.e., block belt, undercord embedded reinforcement, and present day belt with oriented textile reinforcements), is that their performance as expressed in terms of horsepower multiplier is not greater than the 1.6 for the present day belt as based on 12.5 horsepower at 1750 rpm on two 4.75 pitch diameter sheaves for 100 hours.